A laminated antenna duplexer and a dielectric filter

Abstract: A dielectric antenna duplexer used in a high frequency radio device such as a portable telephone, and a dielectric filter for forming the duplexer of the SIR (stepped impedance resonator) composed by cascade connection of first transmission lines having one end grounded and second transmission lines having one end open and lower in characteristic impedance than in the first transmission lines, first transmission lines and second transmission lines are individually coupled in electromagnetic field, thereby forming an antenna duplexer and a dielectric filter of small insertion loss, high bandwidth selectivity, excellent band pass characteristic, and low cost.

Description: This invention relates to a dielectric antenna duplexer and a dielectric filter used mainly in high frequency radio devices such as mobile telephones. An antenna duplexer is a device for sharing one antenna by a transmitter and a receiver, and it is composed of a transmission filter and a reception filter. The invention is particularly directed to a laminated dielectric antenna duplexer having a laminate structure by laminating a dielectric sheet and an electrode layer and baking into one body. It also related to a laminated dielectric filter. The invention is further directed to a block type dielectric filter applying a circuit construction of the laminated dielectric filter of the invention into a conventional dielectric block structure. Along with the advancement of mobile communications, recently, the antenna duplexer is used widely in many hand-held telephones and car-mounted telephones. An example of a conventional antenna duplexer is described below with reference to a drawing. Fig. 20 is a perspective exploded view of a conventional antenna duplexer. In Fig. 20, reference numerals 701 to 706 are dielectric coaxial resonators, 707 is a coupling substrate, 708 is a metallic case, 709 is a metallic cover, 710 to 712 are series capacitors, 713 and 714 are inductors, 715 to 718 are coupling capacitors, 721 to 726 are coupling pins, 731 is a transmission terminal, 732 is an antenna terminal, 733 is a reception terminal, and 741 to 747 are electrode patterns formed on the coupling substrate 707. The dielectric coaxial resonators 701, 702, 703, series capacitors 710, 711, 712, and inductors 713, 714 are combined to form a transmission band elimination filter. The dielectric coaxial resonators 704, 705, 706, and coupling capacitors 715, 716, 717, 718 compose a reception band pass filter. One end of the transmission filter is connected to a transmission terminal which is electrically connected with a transmitter, and the other end of the transmission filter is connected to one end of a reception filter, and is also connected to an antenna terminal electrically connected to the antenna. The other end of the reception filter is connected to a reception terminal which is electrically connected to a receiver. The operation of an antenna duplexeris described below. First of all, the transmission band elimination filter shows a small insertion loss to the transmission signal in the transmission frequency band, and can transmit the transmission signal from the transmission terminal to the antenna terminal while hardly attenuating it. By contrast, it shows a larger insertion loss to the reception signal in the reception frequency band, and reflects almost all input signal in the reception frequency band, and therefore the reception signal entering from the antenna terminal returns to the reception band pass filter. On the other hand, the reception band filter shows a small insertion loss to the reception signal in the reception frequency band, and transmits the reception signal from the antenna terminal to the reception terminal while hardly attenuating it. The transmission signal in the transmission frequency band shows a large insertion loss, and reflects almost all input signal in the transmission frequency band, so that the transmission signals coming from the transmission filter is sent out to the antenna terminal. In this design, however, in manufacturing dielectric coaxial resonators, there is a limitation in fine processing of ceramics, and hence it is hard to reduce its size. Downsizing is also difficult because many parts are used such as capacitors and inductors, and another problem is the difficulty in lowering the assembling cost. The dielectric filter is a constituent element of the antenna duplexer, and is also used widely as an independent filter in mobile telephones and radio devices, and there is a demand that they be smaller in size and higher in performance. Referring now to a different drawing, an example of a conventional block type dielectric filter possessing a different constitution from the above described structure is described below. Fig. 21 is a perspective oblique view of a block type dielectric filter of the prior art. In Fig. 21, reference numeral 1200 is a dielectric block, 1201 to 1204 are penetration holes, and 1211 to 1214, and 1221, 1222, 1230 are electrodes. The dielectric block 1200 is entirely covered with electrodes, including the surface of the penetration holes 1201 to 1204, except for peripheral parts of the electrodes on the surface of which the electrodes 1221, 1222 and others are formed. The operation of the thus constituted dielectric filter is described below. The surface electrodes in the penetration holes 1201 to 1204 serve as the resonator, and the electrode 1230 serves as the shield electrode. The electrodes 1211 to 1214 are to lower the resonance frequency of the resonator composed of the electrodes in the penetration holes, and functions as the loading capacity electrode. By nature, a 1/4 wavelength front end short-circuit transmission line is not coupled at the resonance frequency and shows a band stop characteristic, but by thus lowering the resonance frequency, an electromagnetic field coupling between transmission lines occurs in the filter passing band, so that a band pass filter is created. The electrodes 1221, 1222 are input and output coupling capacity electrodes, and input and output coupling is effected by the capacity between these electrodes and the resonator, and the loading capacity electrode. The operating principle of this filter is a modified version of a comb-line filter disclosed in the literature (for example, G.L. Matthaei, "Comb-Line Band-pass Filters of Narrow or Moderate Bandwidth"; the Microwave Journal, August 1963). The block type filter in this design is a comb-line filter composed of a dielectric ceramic (for example, see U. S. Patent 4,431,977). The comb-line filter always requires a loading capacity for lowering the resonance frequency in order to realize the band pass characteristic. Fig. 22 shows the transmission characteristic of the comb-line type dielectric filter in the prior art. The transmission characteristic shows the Chebyshev characteristic increasing steadily as the attenuation outside the bandwidth departs from the center frequency. In this construction, however, it is not possible to realize the elliptical function characteristic possessing the attenuation pole near the bandwidth of the transmission characteristic, and hence the range of selection is not sufficient for filter performance. Also, in such dielectric filter, for smaller and thinner constitution, the flat type laminate dielectric filter that can be made thinner than the coaxial type is expected henceforth, and several attempts have been made to design such a device. A conventional example of a laminated dielectric filter is described below. The following explanation relates to a laminated "LC filter" (trade mark) that is put into practical use as a laminated dielectric filter by forming lumped element type capacitors and inductors in a laminate structure. Fig. 23 is a perspective exploded view showing the structure of a conventional laminate "LC filter". In Fig. 23, reference numerals 1 and 2 are thick dielectric layers. On a dielectric sheet 3 are formed inductor electrodes 3a, 3b, and capacitor electrodes 4a, 4b are formed on a dielectric sheet 4, capacitor electrodes 5a, 5b on a dielectric sheet 5, and shield electrodes 7a, 7b on a dielectric sheet 7. By stacking up all these dielectric layers and dielectric sheets together with a dielectric sheet 6 for protecting the electrodes, an entirely laminated structure is formed. The operation of the thus constituted dielectric filter is described below. First, the confronting capacitor electrodes 4a and 5a, and 4b and 5b respectively compose parallel plate capacitors. Each parallel plate capacitor functions as a resonance circuit as connected in series to the inductor electrodes 3a, 3b through side electrodes 8a, 8b. Two inductors are coupled magnetically. The side electrode 8b is a grounding electrode, and the side electrode 8c is connected to terminals 3c, 3d connected to the inductor electrode to compose a band pass filter as input and output terminals (for example, JP-A-3-72706(1991)). In such a constitution, however, when the inductor electrodes are brought closer to each other to narrow the interval in order to reduce in its size, the magnetic field coupling between the resonators becomes too large, and it is hard to realize a favorable band pass characteristic narrow in the bandwidth. It is moreover difficult to heighten the unloaded Q value of the inductor electrodes, and hence the filter insertion loss is large. Another different conventional example of a laminated dielectric filter is described below with reference to an accompanying drawing. Fig. 24(a) and (b) shows the structure of a conventional laminated dielectric filter. In Fig. 24(a) and (b), 1/4 wavelength strip lines 820, 821 are formed on a dielectric substrate 819. Input and output electrodes 823, 824 are formed on the same plane as the strip lines 820, 821. The strip line 820 is composed of a first portion 820a (L 1 indicates the length of 820a) having a first line width W 1 (Z 1 indicates the characteristic impedance of W 1 ) confronting the input and output electrodes 823, a second portion 820b (L 2 indicates the length of 820b) having a second line width W 2 narrower than the first line width W 1 , and a third portion 820c having a third line width narrower than the first line width W 1 but broader than the second line width W 2 (Z 2 indicates the characteristic impedance of W 2 ). Similarly, the strip line 821 is composed of a first portion 821a having a first line width W 1 confronting the input and output electrodes 824, a second portion 821b having a second line width W 2 narrower than the first line width W 1 , and a third portion 821c having a third line width narrower than the first line width W 1 but broader than the second line width W 2 . The strip lines 820, 821 are connected with a short-circuit electrode 822, and the resonator 801b is in a pi-shape. A dielectric substrate 819 is covered by grounding electrodes 825, 826 at both surfaces. At one side 819a, side electrodes 827,828 are formed, and the grounding electrodes 825, 826, and short-circuit electrodes 822 are connected. On the other side 819b, side electrodes to be connected with the input and output electrodes 823, 824 respectively are formed. The strip lines 820, 821 are capacitively coupled with the input and output electrodes 823, 824, respectively, thereby constituting a filter as described for example, in U. S. Patent 5,248,949. In such constitution, however, same as the conventional block type dielectric filter, the elliptical function characteristic possessing the attenuation pole near the passing band of the transmission characteristic cannot be realized, and hence the scope of performance of the filter is not wide enough. In view of the above-mentioned problems, it is hence a primary object of the invention to provide an antenna duplexer and dielectric filter at low cost which has an excellent band pass characteristic with small insection loss and high bandwidth selectivity. Another object is to provide a laminated dielectric antenna duplexer and laminate dielectric filter having a small and thin flat structure. It is a further object of the invention to provide a block type dielectric filter having low insection cost, possessing low insection loss and high band width selectivity and having the same circuit constitution as in the laminated dielectric filter described above. In order to accomplish these and other objects and advantages, the first case of this invention provides a dielectric filter as defined in claim 1. A filter according to the preamble of claim 1 is known from document GB-A-2163606. According to the specified constitution, in the dielectric filter of the invention, not only is the resonator length shortened by the SIR structure, but also the passing band and attenuation pole can be freely formed at the designed frequency, so that a superior degree of selectivity is realized in a small size. It is preferable that the open end of the TEM mode resonator is grounded with an electrical capacity. It is preferable that the TEM mode resonators and input and output terminals are coupled capacitively. In the dielectric filter of those embodiments, the resonance frequency can be further lowered by the loading capacity, and the resonator line length is shortened, so that the filter may be further reduced in size. In the capacitive coupling method, the filter can be reduced in size because the magnetic field coupling line in the conventional comb-line filter is not necessary. Further, because of capacitive coupling at the open end, a small coupling capacity is sufficient. It is preferable that the attenuation pole frequency of the transmission characteristic is adjusted by varying the line distance of the first transmission lines and the line distance of the second transmission lines. In the dielectric filter of this embodiment, by adjusting the even/odd mode impedance ratio of the transmission line by the distance between lines, the degree of coupling can be changed only by changing the electrode pattern, and it is easy to realize, and it is free from deterioration of unloaded Q value of the resonator. It is preferable that the line length of the first transmission lines and the line length of the second transmission lines are equalized. In the dielectric filter of this embodiment, by equalizing the line length of each transmission line of the SIR, not only can the resonator length be set to the shortest possible distance, but also a very complicated design formula can be summed up in a simple form, making it possible to design analytically. It is preferable that the TEM mode resonator is comprised of an integrated coaxial resonator formed of a penetration hole provided in a dielectric block. It is preferable that the TEM mode resonator is comprised of a strip line resonator formed on a dielectric sheet. In the dielectric filter of the invention, when a block type coaxial resonator is used, it is easy to manufacture by pressing and baking the dielectric ceramic, and materials of high baking temperature and high dielectric constant can be selected, and the filter can be reduced in size. Additionally, since the unloaded Q value is high, the insertion loss can be reduced. On the other hand, when a strip line resonator is used, the thickness can be significantly reduced owing to the flat structure. It is preferable that the value of dividing the even mode impedance by the odd mode impedance of the first transmission lines is set larger than the value of dividing the even mode impedance by the odd mode impedance of the second transmission lines. It is preferable that the value of dividing the even mode impedance by the odd mode impedance of the first transmission lines is set smaller than the value of dividing the even mode impedance by the odd mode impedance of the second transmission lines. In the dielectric filter of the invention as set forth in those embodiments, when the even/odd mode impedance ratio of the first transmission line is smaller than the even/odd mode impedance ratio of the second transmission line, a band pass filter possessing an attenuation pole at the low attenuation band (low-zero filter) can be made. Furthermore, when the even/odd mode impedance ratio of the first transmission line is larger than the even/odd mode impedance ratio of the second transmission line, a band pass filter possessing an attenuation pole at the high attenuation band (high-zero filter) can be made. It is preferable that the value of dividing the even mode impedance of the second transmission lines by the even mode impedance of the first transmission lines is set at 0.2 or more and 0.8 or less. It is preferable that the value of dividing the even mode impedance of the second transmission lines by the even mode impedance of the first transmission lines is set at 0.4 or more and 0.6 or less. In the dielectric filter of the invention, by setting the even mode impedance ratio at 0.2 to 0.8, preferably 0.4 to 0.6, both the magnitude of the line width and gap that can be actually manufactured, and the shortening of the resonator length can be achieved at the same time, and manufacturing is made easier. It is preferable that the TEM mode resonators are capacitively coupled by capacity coupling means provided separately, and coupling of the TEM mode resonators is achieved by a combination of electromagnetic field coupling and capacity coupling. It is preferable that the capacity coupling by the capacity coupling means is achieved in the second transmission lines. It is also preferable that the capacity coupling by the capacity coupling means is achieved at the open end of the TEM mode resonator. For this specific constitution of the first invention, the following features which are similar to those mentioned above are also provided. It is preferable that the open end of the TEM mode resonator is grounded through the capacity. In addition, it is preferable that the TEM mode resonators and input and output terminals are coupled capacitively. In the dielectric filter of the invention, an attenuation pole can be generated very closely to the passing band of the transmission characteristic, and the resonator line length can be further shortened, so that a dielectric filter of small size having a high selectivity can be realized. It is preferable that the attenuation pole frequency of the transmission characteristic is adjusted by varying the line distance of the first transmission lines and the line distance of the second transmission lines. In the laminated dielectric filter of the invention, by adjusting the even/odd mode impedance ratio of the transmission line, the degree of coupling can be adjusted by only changing the electrode pattern, and it is easy to realize. Also, the unloaded Q value of the resonator does not deteriorate. It is preferable that the line length of the first transmission lines and the line length of the second transmission lines are equalized. In the laminated dielectric filter of the invention as set forth in the embodiment, by equalizing the line length of each transmission line of the SIR, not only can the resonator length be set to the shortest possible distance, but also a very complicated design formula can be summed up in a simple form, making it possible to design analytically. It is preferable that the TEM mode resonator is comprised of an integrated coaxial resonator formed of a penetration hole provided in a dielectric block. It is preferable that the TEM mode resonator is comprised of a strip line resonator formed on a dielectric sheet. In the dielectric filter of the invention, when a block type coaxial resonator is used, it is easy to manufacture by pressing and baking the dielectric ceramic, and materials of high baking temperature and high dielectric constant can be selected, and the filter can be reduced in size, and moreover, since the unloaded Q value is high, the insertion loss can be reduced. On the other hand, when a strip line resonator is used, the thickness can be significantly reduced owing to the flat structure. It is preferable that the value of dividing the even mode impedance by the odd mode impedance of the first transmission lines is set larger than the value of dividing the even mode impedance by the odd mode impedance of the second transmission lines. It is preferable that the value of dividing the even mode impedance by the odd mode impedance of the first transmission lines is set smaller than the value of dividing the even mode impedance by the odd mode impedance of the second transmission lines. In the dielectric filter of the invention as set forth in those embodiments, by setting the even/odd mode impedance ratio of the first transmission line smaller or larger than the even/odd mode impedance ratio of the second transmission line, a band pass filter of low-zero or of high zero can be freely composed. It is preferable that the attenuation pole of transmission characteristic is formed in a frequency range of within 15% on both sides of the polarity of the center frequency. In the dielectric filter of the invention as set forth in the embodiment, a filter having a high selectivity can be realized. It is preferable that the value of dividing the even mode impedance of the second transmission lines by the even mode impedance of the first transmission lines is set at 0.2 or more and 0.8 or less. It is preferable that the value of dividing the even mode impedance of the second transmission lines by the even mode impedance of the first transmission lines is set at 0.4 or more and 0.6 or less. In the dielectric filter of the invention by setting the even mode impedance ratio at 0.2 to 0.8, preferably 0.4 to 0.6, both the magnitude of the line width and gap that can be actually manufactured, and the shortening of the resonator length can be achieved at the same time, and manufacturing is made easier. Fig. 1 is a perspective exploded view of a laminated dielectric filter in a first embodiment of the invention. Fig. 2 is an equivalent circuit diagram of the laminated dielectric filter in the first embodiment of the invention. Fig. 3 is a graph showing the relationship between the even mode impedance step ratio and normalized resonator line length in the laminated dielectric filter in the first embodiment of the invention. Fig. 4 is a graph showing the relationship between the even mode impedance step ratio and even/odd mode impedance ratio in the laminated dielectric filter in the first embodiment of the invention. Fig. 5 is a graph showing the relationship between the even mode impedance and even/odd mode impedance ratio to the structural parameters of a parallel coupling strip line of the invention. Fig. 6 (a) and (b) are graphs showing simulation results of design value of transmission characteristic of the laminated dielectric filter in the first embodiment of the invention, Fig. 6 (a) showing the characteristic of a first trial filter with a low-zero, and Fig. 6 (b) showing the characteristic of a second trial filter with a high-zero. Fig. 7 (a) and (b) are graphs showing the measured value and calculated value of transmission characteristic of the laminated dielectric filter in the first embodiment of the invention, Fig. 7 (a) showing the characteristic of a first trial filter with a low-zero, and Fig. 7 (b) showing the characteristic of a second trial filter with a high-zero. Fig. 8 is a perspective view of a modified form of laminated dielectric filter in the first embodiment of the invention. Fig. 9 (a) is a perspective oblique view of a block type dielectric filter in a second embodiment of the invention, and Fig. 9 (b) is a sectional view on plane A-A' of the invention. Fig. 10 is a perspective exploded view of a laminated dielectric filter in a third embodiment of the invention. Fig. 11 is a graph showing the relationship between the loading capacity and the normalized resonator line length in the laminated dielectric filter in the third embodiment of the invention. Fig. 12 is a perspective exploded view of a laminated dielectric filter in a fourth embodiment of the invention. Fig. 13 is an equivalent circuit diagram of the laminated dielectric filter in the fourth embodiment of the invention. Fig. 14 (a) and (b) are graphs showing the relation between the attenuation frequency and even/odd mode impedance ratio of the laminated dielectric filter in the fourth embodiment of the invention, Fig. 14 (a) showing the case for a low-zero filter and Fig. 14 (b) showing the case for a high-zero filter. Fig. 15 is a graph showing the relationship of the coupling capacity, the even/odd mode impedance ratio, and normalized resonator line length of the laminated dielectric filter in the fourth embodiment of the invention. Fig. 16 is a graph showing the relationship of the loading capacity, even/odd mode impedance ratio, and normalized resonator line length of the laminated dielectric filter in the fourth embodiment of the invention. Fig. 17 (a) and (b) are graphs showing the relationship of the attenuation frequency, coupling capacity, and loading capacity of the laminated dielectric filter in the fourth embodiment of the invention, Fig. 17 (a) showing the case for a low-zero filter and Fig. 17 (b) showing the case for a high-zero filter. Fig. 18 (a) and (b) are graphs showing the simulation results of transmission characteristic of the laminated dielectric filter of the first embodiment and the laminated dielectric filter in the fourth embodiment of the invention, Fig. 18 (a) showing the characteristic of the low-zero filter and Fig. 18 (b) showing the characteristic of the the high-zero filter. Fig. 19 (a) is a perspective view of a block type dielectric filter in a fifth embodiment of the invention, and Fig. 19 (b) is a sectional view of section A-A' in Fig. 19 (a). Fig. 20 is a perspective exploded view of a dielectric antenna duplexer of the prior art. Fig. 21 is a perspective view of a block dielectric filter of the prior art. Fig. 22 is a graph showing transmission characteristic and reflection characteristic of a comb-line dielectric filter of the prior art. Fig. 23 is a perspective exploded view of a laminated LC filter of the prior art. Fig. 24 (a) and (b) is a perspective view of a laminated dielectric filter of the prior art. An antenna duplexer is comprises a combination of a transmission filter and a reception filter. In the following illustrative examples, first, the individual filters which are used in the antenna duplexer, particularly the laminated and block dielectric filters are described, and then the laminated antenna duplexers using such filters are described. EXAMPLE 1 A laminated dielectric filter in a first embodiment of the invention is described below with reference to the drawings. Fig. 1 is a perspective view of a dielectric filter in the first embodiment of the invention. In Fig. 1, reference numerals 10a, 10b are thick dielectric sheets. Strip line resonator electrodes 11a, 11b are formed on the dielectric sheet 10a, and capacity electrodes 12a, 12b are formed on the dielectric sheet 10c. The strip line resonator electrodes 11a, 11b have a SIR (stepped impedance resonator) structure in which the overall line length is shorter than a quarter wavelength composed by the cascade connection of the other ends of first transmission lines 17a, 17b with high characteristic impedance grounded at one end, and second transmission lines 18a, 18b with low characteristic impedance opened at one end. The SIR structure is described in M. Makimoto et al., "Compact Bandpass Filters Using Stepped Impedance Resonators," Proceedings of the IEEE, Vol. 67, No. 1, pp. 16-19, January 1979 and is disclosed in U.S. Patent No. 4,506,241 which are incorporated by reference. It is known in the art that the line length of the resonator can be cut shorter than a quarter wavelength. By contrast, the structure of the invention differs greatly from the prior art in that each resonator has the SIR structure, and the first transmission lines are mutually coupled electromagnetically, and the second transmission lines are mutually coupled electromagnetically, with each electromagnetic field coupling amount set independently by varying the line distance of the transmission lines. The short-circuit end side of the first transmission line is grounded through a common grounding electrode 16. By grounding through the common grounding electrode 16, grounding is done securely, and fluctuations in the resonance frequency due to cutting errors when cutting off the dielectric sheet can be decreased. The strip line resonator electrodes 11a, 11b and input and output terminals 14a, 14b are coupled capacitively through the capacity electrodes 12a, 12b at the open ends of the strip line resonator electrodes. In the capacitive coupling method, as compared with the magnetic field coupling method generally employed in comb-line filters, since the coupling line is not necessary, the filter can be reduced in size. Application of the capacitive coupling method in this filter structure is accomplished for the first time by the establishment of the design method mentioned below. Another feature is that only a small capacity is enough for the coupling capacity because of coupling at open ends. A shield electrode 13a is formed on the dielectric sheet 10b, and a shield electrode 13b is formed on the dielectric sheet 10d. Each shield electrode is grounded by the grounding terminals 15a, 15b, 15c, 15d formed on the side electrodes. In the structure of the invention, the entire filter is covered with the shield electrodes, and hence the filter characteristic is hardly affected by external effects. By laminating the dielectric sheet 10e for electrode protection and laminating all other dielectric sheets, an entirely laminated structure is formed. Using a dielectric material of, for example, Bi-Ca-Nb-O ceramics with dielectric constant of 58 disclosed in H. Kagata et al.: "Low-fire Microwave Dielectric Ceramics and Multilayer Devices with Silver Internal Electrode," Ceramic Transactions, Vol. 32, The American Ceramic Society Inc., pp. 81-90, or other ceramic materials that can be baked at 950 degrees C or less, a green sheet is formed, and an electrode pattern is printed with metal paste of high electric conductivity such as silver, copper and gold, thereby laminating and baking integrally. In this way, when the laminate structure is formed by using the strip line resonators, the thickness can be reduced significantly. Operation of the thus constituted dielectric filter is described by reference to Fig. 1 and Fig. 2. Fig. 2 shows an equivalent circuit diagram of the dielectric filter in the first embodiment. The filter transmission characteristic in Fig. 2 can be calculated by using the even/odd mode impedance of the parallel coupling transmission line. In Fig. 2, reference numerals 21, 22 are input and output terminals, 17a, 17b are first transmission lines of the strip line resonator, 18a, 18b are second transmission lines of the strip line resonator, and capacitors 23, 24 are input and output coupling capacitors located between the strip line resonator electrodes 11a, 11b, and capacity electrodes 12a, 12b. In the case of a two-stage filter or a two-pole filter, the filter designing method in the first embodiment of the invention is described below. The even/odd mode impedances of the first transmission lines are supposd to be Z e1 , Z o1 , and the even/mode impedances of the second transmission lines to be Z e2 , Z o2 . The four-port impedance matrix of each transmission line is given in formula (1) by referring to, for example, the literature (T. Ishizaki et al., "A Very Small Dielectric Planar Filter for Portable Telephones": 1993 IEEE MTT-S, Digest H-1). Therefore, the two-port admittance matrix of two-terminal pair circuit 25 is newly calculated as in formula (2) for the structure of the invention, by connecting them in cascade, grounding one end, and using the other end as an input and output terminal. However, the line length of the first transmission lines and second transmission lines is set at the same line length L. By equalizing the line length, not only can the resonator length be set to the shortest, but also a very complicated calculation formula can be summarized into a simple form, thereby making it possible to design analytically. K e , H o , α, β, and t' are defined in formula (3). Where L is the line length of first transmission line or second transmission line, c is the velocity of light, and k is the propagation velocity ratio. To design a filter, first, from the design specification, the center frequency f o , attenuation pole frequency f p , bandwidth bw, and in-band ripple L r are determined. From these values, the value of g necessary for filter design is determined, and therefore the interstage admittance Y 3 and the shunt admittance of the modified admittance inverter Y 01 e , and input and output coupling capacities (C 01 ) 23, 24 are determined. Calculation of g, Y 3 , Y 01 e ,C 01 is shown in the literature (G.L. Matthaei et al., "Microwave Filters, Impedance-Matching Networks, and Coupling Structures": McGraw-Hill, 1964). Herein, t' in formula (3), replacing f with f o or f p , is defined as t' o , t' p . Therefore, the formulas necessary for realizing the filter characteristic to be designed are formula (4) for giving the attenuation pole frequency f p , formula (5) for giving the filter center frequency f o , and formula (6) for giving the interstage admittance Y 3 . The solution that satisfies these three formulas simultaneously is the design value of the dielectric filter in Example 1 of the invention. Next, considering the structural parameters of the strip line, Z e1 and Z e2 , that is, Z e1 and K e (=Z e2 /Z e1 ) are properly determined. From formula (2) and formula (3), β can be eliminated, and t' o and t' p are determined. Hence, the line length L of each transmission line is determined. If the loading capacity is present at the open end of the strip line, formula (5) can be changed to formula (7) in the filter design formula. where Y L is the admittance due to loading capacity. A design example of the filter of the embodiment is shown. Table 1 shows circuit parameter design values, with the center frequency f o of 1000 MHz, bandwidth bw of 50 MHz, in-band ripple L r of 0.2 dB, and attenuation pole frequency f p of 800 MHz in a first trial filter, and 1200 MHz in a second trial filter. Circuit parameter design values First filter Second filter Z e1 20Ω 20Ω Z 01 18.46Ω 14.88Ω Z e2 10Ω 10Ω Z 02 7.02Ω 7.41Ω L 3.00mm 3.20mm C 01 1.34pF 1.34pF Herein, the dielectric constant of the dielectric sheet is 58, and hence k is 0.131, Z e1 is 20Ω, and K e is 0.5. The loading capacity due to the discontinuous part at the open end is estimated at 3 pF. For an arbitrary value of the even mode impedance step ratio K e , the relation between K e and normalized resonator line length S is as shown in Fig. 3. The normalized resonator line length S is the value of the resonator line length of the filter divided by a quarter wavelength of the propagation wavelength. In the filter of the embodiment, in this way, by designing the resonator in the SIR structure, the line length can be set shorter than the quarter wavelength if loading capacity is not available, so that the filter can be reduced in size. That is, the resonator line length is shorter when the even mode impedance step ratio K e is smaller. Moreover, the relation of K e with the even/odd mode impedance ratio P 1 (=Z e1 /Z o1 ) of the first transmission line and the even/odd mode impedance ratio P 2 (=Z e2 /Z o2 ) of the second transmission line is shown in Fig. 4. The larger the value of K e , the larger the even/odd mode impedance ratio P 2 of the second transmission line, and hence the gap between the strip line resonators must be decreased, which is more difficult. On the other hand, if K e is small, the even mode impedance Z e1 of the first transmission line is considerably high, and the line width of the strip line may be narrower, which is also difficult to accomplish. To realize a favorable filter characteristic in the constitution of the embodiment, as determined from Fig. 4, the even/odd mode impedance ratio P 1 of the first transmission line and the even/odd mode impedance ratio P 2 of the second transmission line must be 1.05 or more and 1.1 or more respectively. Fig. 5 is a design chart for explaining the relation between the even mode impedance Z e and even/odd mode impedance ratio P as the parameter of strip line structure. In Fig. 5, at the dielectric constant of 58, the thickness of the dielectric sheet between strip line and upper and lower shield electrodes of 0.8 mm respectively, is calculated by varying the line width w of the strip line from 0.2 mm to 2.0 mm, and the gap between parallel strip lines from 0.1 mm to 2.0 mm. Fig. 5 enables checking whether the even/odd mode impedance ratio P of the transmission lines in Fig. 4 can be obtained. As a result, the value of the structural parameter for realizing the circuit parameter in Table 1 is determined as shown in Table 2 by referring to Fig. 5. Structural parameter design values First filter Second filter W 1 0.35mm 0.44mm g 1 1.22mm 0.54mm W 2 1.55mm 1.51mm g 2 0.20mm 0.27mm In the design in Table 2, the even/odd mode impedance ratio P of the transmission line is adjusted by varying the line distance, that is, the gap g. The coupling degree adjustment by the line distance is possible only by varying the electrode pattern, and it is easier to realize by far as compared with the method of, for example, varying the thickness of the dielectric sheet, and it is advantageous that the unloaded Q value of the resonator does not deteriorate. Fig. 6 is a graph showing the simulation results of the design value of transmission characteristic of the dielectric filter in the first embodiment. Fig. 7 shows the characteristic of the trial production of the filter of the embodiment, in which the solid line shows the measured value, and the broken line shows the calculated value about the actual dimensions of the trial product. In both diagrams, (a) shows the characteristic of the first trial filter with a low-zero, and (b) shows the characteristic of the second trial filter with a high-zero. These diagrams indicate that an attenuation pole is generated at the design frequency. The invention attains a novel effect of realizing superior selectivity by mutual electromagnetic coupling of the first transmission lines and second transmission lines of the resonator of the SIR structure, thereby not only shortening the resonator length, but also forming an attenuation pole at the design frequency. Thus, according to the embodiment, at least two or more TEM mode resonators are comprised in the SIR (stepped impedance resonator) structure with the overall line length shorter than a quarter wavelength constituted by cascade connection of other ends of the first transmission lines having one end grounded and the second transmission lines having one end open with the characteristic impedance lower than that of the first transmission lines. The first transmission lines are coupled electromagnetically, and the second transmission lines are coupled electromagnetically, and both electromagnetic field coupling amounts are set independently, and therefore a passing band and an attenuation pole are generated in the transmission characteristic, thereby realizing a small dielectric filter having a high selectivity. In this embodiment, a strip line resonator is shown, but a resonator of any structure may be used as far as it is a TEM mode resonator, and it is the same in the following examples. A laminated dielectric filter in a modified Example 1 of the invention is described below with reference to a drawing. Fig. 8 is a perspective exploded view of the laminated dielectric filter showing a modified first example of the invention. In Fig. 8, those same as the constitution in Fig. 1 are identified with the same reference numerals. The operating principle of this embodiment is the same as in the first embodiment. This embodiment differs from the first embodiment shown in Fig. 1 in that capacity electrodes 29a, 29b are formed on the dielectric sheet 10a, the same as the strip line resonator electrode layer. Accordingly, the dielectric sheet 10c in the first embodiment is not necessary, and the number of times of printing of the electrodes can be reduced by one, and it is free from the control of the thickness of the dielectric sheet 10c which is a cause of fluctuation in filter characteristic. Moreover, by forming a capacitor comprised of a capacity electrode as an interdigital type capacitor, a large capacity can be obtained easily, so that a wide range characteristic can be also realized. Example 2 A block type dielectric filter in an embodiment of the invention is described below with reference to the drawings. Fig. 9 (a) is a perspective oblique view of the block type dielectric filter showing the second embodiment of the invention, and Fig. 9 (b) is a sectional view of section A-A' of the block type dielectric filter showing the second embodiment of the invention. The example differs from Example 1 in that the block coaxial resonator formed in the penetration hole of the dielectric block is used instead of the strip line resonator as the TEM mode resonator. In Fig. 9 (a) and (b), reference numeral 1010 denotes a dielectric block, 1011, 1012, 1013, 1014 are resonator electrodes, 1015, 1016 are input and output coupling capacity electrodes, and 1017 is a shield electrode. The resonator electrodes are individually composed of first transmission lines 1031, 1032, 1033, 1034 of high characteristic impedance, and second transmission lines 1021, 1022, 1023, 1024 of low characteristic impedance, and they are mutually coupled in an electromagnetic field. The magnitude of the electromagnetic field coupling can be adjusted by varying the distance between the transmission lines, or shaving off the dielectric by forming a notch or small hole in the dielectric block. In the example, aside from the same effects as in Example 1 by using a coaxial resonator, it is sufficient to press and bake the dielectric ceramic, and hence it is easy to manufacture. Also, since a ceramic material having high baking temperature can be used, materials of high dielectric constant can be used, and the filter may be reduced in size. In addition, since the unloaded Q value is slightly higher than in the strip line resonator, the insertion loss of the filter can be decreased. Example 3 A laminated dielectric filter in an embodiment of the invention is described below with reference to a drawing. Fig. 10 is a perspective exploded view of the laminated dielectric filter. In Fig. 10, those structure that are the same as in Fig. 1 are identified with same reference numerals. What differs from Fig. 1 is that a loading capacity electrode 19 is provided so as to confront the open end portion of the strip line resonator electrodes 11a and 11b. In this embodiment, the resonance frequency can be further lowered by inserting the loading capacitor parallelly to the strip line resonator. As the filter design formula in this embodiment, formula (4) and formula (6) are the same as in Example 1, and only formula (5) is changed to the above described formula (7). Fig. 11 is a graph for explaining the relation between the loading capacity and resonator line length in the third embodiment. By adding the loading capacity, it is known that the resonator line length is further shortened, Thus, by providing the loading capacity electrode 19 confronting the open end portion of the strip line resonator electrodes 11a and 11b, the length of the resonator line can be further shortened, and the filter size can be reduced. Example 4 A laminated dielectric filter in an embodiment of the invention is described below referring to the drawings. Fig. 12 is a perspective exploded view of the laminated dielectric showing the fourth embodiment of the invention. Fig. 13 is an equivalent circuit diagram of the laminated dielectric filter of the fourth embodiment. In Fig. 12, those structures same as in the structures in Fig. 1 are identified with same reference numerals. This embodiment differs from the first embodiment in Fig. 1 in that the coupling capacity electrode 20 and loading capacity electrode 19 are provided confronting the open end portion of the strip line resonator electrodes 11a, 11b. Prior to describing the operation of the dielectric filter of the embodiment, the difficulty in forming the attenuation pole near the passing band in the first embodiment is explained. Fig. 14 (a) and (b) are graphs showing the even/odd mode impedance ratio necessary for the attenuation pole frequency of the dielectric filter in the first embodiment. Fig. 14 (a) shows the filter with a low-zero, and Fig. 14 (b) shows the filter with a high-zero. As the attenuation pole frequency approaches the center frequency, the required even/odd mode impedance ratios P 1 , P 2 become larger. As the guideline for manufacture of actual filter, supposing the minimum value of the manufacturable line width w and gap g to be 0.2 mm, and their maximum value due to the request of the size of the filter to be 2 mm, the even mode impedance Z e that can be realized is in the range of 7 Ω to 35 Ω as shown in Fig. 5. That is, the minimum even mode impedance step ratio K e is 0.2. Moreover, if K e is large, the resonator length cannot be shortened, and hence there is a proper range for K e , and in relation to the structural parameter of the strip line, it is preferably 0.2 to 0.8, and more preferably 0.4 to 0.6. Hence, the even/odd mode impedance ratio P that can be realized is about 1.4 or less when the even mode impedance is 7 Ω, 1.9 or less at 20 Ω, and 2.2 or less at 35 Ω. Limitations on these values are restrictions on how closely the attenuation pole can be brought to the vicinity of the center frequency. In Fig. 14(a) and (b), based on the condition of P 2 being 1.4 or less, in the dielectric filter of the first embodiment, it is determined that the highest frequency of the lower attenuation pole frequency is 814 MHz, and the lowest frequency of the upper attenuation pole frequency is 1154 MHz. To alleviate these limitations, the coupling capacity and loading capacity are introduced, and the result is the dielectric filter of the fourth embodiment of the invention shown in Fig. 12. The operations of the laminated dielectric filter of the fourth embodiment is described referring to Fig. 12 and Fig. 13. The transmission characteristic of the filter in the fourth embodiment shown in Fig. 13 can be calculated the same as in the filter in the first embodiment in Fig. 2 by using the even/odd mode impedance of the parallel coupling transmission line. In Fig. 13, those structures that are the same as in Fig. 2 are identified with the same reference numerals. What differs from Fig. 2 is that a coupling capacity (C c ) 28 formed between coupling capacity electrode 20 and strip line resonator electrodes 11a, 11b, and loading capacities (C L ) 26, 27 formed between the loading capacity electrode 19 and strip line resonator electrodes 11a, 11b are added. Concerning the two-pole filter of the fourth embodiment, a designing method is described below. The two-port admittance of the two-terminal pair circuit 25 of parallel coupling SIR resonator is given in formula (2) as mentioned above. Therefore, in the structure of the embodiment, as the formula necessary for realizing the design filter characteristic, the formulas (4), (5), (6) given in the first embodiment should be rewritten as follows. That is, the formula (8) for giving the attenuation pole frequency f p , the formula (9) for giving the filter center frequency f o , and the formula (10) for giving the interstage admittance Y 3 . The solution that satisfies these three formulas simultaneously is the design value of the dielectric filter of the fourth embodiment of the invention. The relation of the coupling capacity C c of the dielectric filter with a low-zero in the fourth embodiment with the corresponding even/odd mode impedance ratio (P 1 , P 2 ) and normalized resonator line length S is shown in Fig. 15. The relation of the loading capacity C L with the even/odd mode impedance ratio (P 1 , P 2 ) and normalized resonator length S is shown in Fig. 16. These diagrams are calculated at the center frequency f o of 1000 MHz, attenuation pole frequency f p of 800 MHz, and even mode impedance step ratio K e of 0.2. In Fig. 15, the loading capacities (C L ) 26, 27 are fixed at 0 pF, and in Fig. 16 the coupling capacity (C c ) 28 is fixed at 0 pF. When the coupling capacity C c increases, P 1 increases, P 2 decreases, and S is unchanged. On the other hand, when the loading capacity C L increases P 1 decreases, P 2 increases, and S decreases. Therefore, by the combination of the coupling capacity (C c ) 28 and loading capacities (C L ) 26, 27, the even/odd mode impedance ratio (P 1 , P 2 ) can be adjusted to a practical value. Hence, an attenuation pole may be made up in the vicinity of the passing band. Fig. 14 (a), shows that when the even/odd mode impedance ratio P 1 of the first transmission lines is smaller than the even/odd mode impedance ratio P 2 of the second transmission lines, a low-zero is formed in the dielectric filter in the first embodiment. When the even/odd mode impedance ratio P 1 of the first transmission lines is larger than the even/odd mode impedance ratio P 2 of the second transmission lines, Fig.14 (a) shows that a high-zero is formed in the dielectric filter in the first embodiment. On the other hand, Figs. 15, 16 of the fourth embodiment show the possibitity that their relation may be exchanged depending on the magnitude of the coupling capacity and loading capacity. Therefore, by thus properly setting the relation of P 1 and P 2 , the attenuation pole can be freely formed at a specified frequency in the structure of the invention. Fig. 17 (a) is a graph showing the minimum required coupling capacity and loading capacity values for the attenuation pole frequency of the dielectric filter possessing the low-zero in the fourth embodiment. Fig. 17 (b) is a graph showing the minimum required coupling capacity and loading capacity values for the attenuation pole frequency of the dielectric filter with a high-zero in the fourth embodiment. As known from the curves of the graphs, although not created by the dielectric filter of the structure in the first embodiment, the attenuation pole in a frequency range of within 15% on both sides of the polarity of the center frequency, specifically the attenuation pole in a frequency range of 814 MHz to 1154 MHz can be manufactured in the dielectric filter of the structure in the fourth embodiment. It is also shown that the loading capacity is essential in the close vicinity to the passing band. By forming an attenuation pole in the frequency range of within 15% on both sides of the polarity of the center frequency a band pass filter having a high selectivity can be realized. Fig. 18 (a) and (b) are graphs showing the transmission characteristic simulation result for improving the attenuation amount near the passing band of the dielectric filter in the first embodiment and fourth embodiment. Fig. 18 (a) relates to a filter with low-zero, and Fig. 18 (b) shows a filter with a high-zero. In both cases, the solid line shows the characteristic when the attenuation pole is brought closest to the passing band in the filter of the first embodiment, and the broken line shows the characteristic obtained in the filter of the fourth embodiment. In the filter of the fourth embodiment, a superior selectivity characteristic to that of the filter of the first embodiment is obtained. Thus, this embodiment comprises at least two or more TEM mode resonators in the SIR (stepped impedance resonator) structure with an overall line length shorter than a quarter wavelength constituted by cascade connection of other ends of the first transmission lines having one end grounded and the second transmission lines having one end open with the characteristic impedance lower than that of the first transmission lines. The first transmission lines are coupled electromagnetically, and the second transmission lines are coupled electromagnetically. Both electromagnetic coupling amounts are set independently, while at least two TEM mode resonators are capacitively coupled through separate coupling means, so that an attenuation pole can be generated near the passing band of transmission characteristic, which is an excellent characteristic. Also, in the fourth embodiment, by inserting the loading capacity parallelly to the strip line resonator, the resonator line length can be further shortened, and therefore the filter can be reduced in size. Therefore, a small dielectric filter with high selectivity can be realized. Such characteristic is very preferable for a high frequency filter for use in, for example, a portable telephone. Example 5 A block type dielectric filter in an embodiment of the invention is described below referring to the drawings. Fig. 19 (a) is a perspective oblique view of the block type dielectric filter showing the fifth embodiment of the invention, and Fig. 19 (b) is a sectional view of section A-A' of the block type dielectric filter showing the fifth embodiment of the invention. The fifth embodiment differs from the fourth embodiment in that an integrated coaxial resonator formed through a penetration hole of the dielectric block is used instead of the strip line resonator, as the TEM mode resonator. In Fig. 19 (a) and (b), those same structures as in the constitution in Fig. 9 are identified with same reference numerals. Reference numeral 1010 is a dielectric block, 1011, 1012, 1013, 1014 are resonator electrodes, 1015, 1016 are input and output coupling capacity electrodes, 1017 is a shield electrode, and 1018a, 1018b, 1018c are coupling capacity electrodes. The resonator electrodes are respectively composed of first transmission lines 1031, 1032, 1033, 1034 of high characteristic impedance, and second transmission lines 1021, 1022, 1023, 1024 of low characteristic impedance, and they are mutually coupled electromagnetically. Capacitive coupling is effected by the capacity in the gaps of the coupling capacity electrodes 1018a, 1018b, and 1018c. The magnetitude of the electromagnetic field coupling can be adjusted by varying the distance between transmission lines, or shaving off the dielectric by forming a notch or a tiny hole in the dielectric block. In the fifth embodiment, aside from the same effects as in the fourth embodiment, by using the integrated coaxial resonator, it is sufficient to press, form and bake the dielectric ceramic, and it is easy to manufacture. Ceramic materials of high baking temperature can be used, and hence materials of high dielectric constant can be used. In addition, since the unloaded Q value is slightly higher than in the strip line resonator, the filter insertion loss can be decreased.

Claims: A dielectric filter comprising at least two TEM mode resonators having a stepped impedance resonator structure with a total line length of each of the resonators being shorter than a quarter wavelength of a center frequency of a passband of the filter, the stepped impedance resonator structure comprising a cascade connection of both ends of first transmission line sections (17a, 17b) having characteristic impedances and being grounded at one end, and second transmission line sections (18a, 18b) opened at one end and having characteristic impedances lower than the characteristic impedances of the first transmission line sections, wherein the first transmission line sections are coupled to each other electromagnetically with even-mode impedance Z e1 and odd-mode impedance Z o1 , the second transmission line sections are coupled to each other electromagnetically with even-mode impedance Z e2 and odd-mode impedance Z o2 , and wherein a ratio P 1 defined as Z e1 divided by Z o1 and a ratio P 2 defined as Z e2 divided by Z o2 , are set independently so as to generate the passband and characterised by an attenuation pole in the transmission characteristic of the filter with the attenuation pole frequency being controlled relative to the center frequency of the passband. The dielectric filter of claim 1, wherein the open end of the TEM mode resonator is grounded by a capacitance. The filter of claim 1 or 2, wherein at least two TEM mode resonators and input and output terminals are coupled capacitively. The filter of claim 1, 2 or 3, wherein the attenuation pole frequency of the transmission characteristic is adjusted by varying the line distance of the first transmission lines and the line distance of the second transmission lines. The filter of any of claims 1 to 4, wherein the first and second transmission lines have a line length equal to each other. The filter of any of claims 1 to 5, wherein the TEM mode resonator is comprised of an integrated coaxial resonator formed of a penetration hole provided in a dielectric block. The filter of any of claims 1 to 5, wherein the TEM mode resonator is comprised of a strip line resonator formed on a dielectric sheet. The filter of any of claims 1 to 7, wherein the value of dividing the even mode impedance by the odd mode impedance of the first transmission lines is set larger than the value of dividing the even mode impedance by the odd mode impedance of the second transmission lines. The filter of any of claims 1 to 7, wherein the value of dividing the even mode impedance by the odd mode impedance of the first transmission lines is set smaller than the value of dividing the even mode impedance by the odd mode impedance of the second transmission lines. The filter of any of claims 1 to 9, wherein the value of dividing the even mode impedance of the second transmission lines by the even mode impedance of the first transmission lines is set at 0.2 to 0.8. The filter of any of claims 1 to 10, wherein the value of dividing the even mode impedance of the second transmission lines by the even mode impedance of the first transmission lines is set at 0.4 to 0.6. The filter of any of claims 1 to 11, wherein at least two TEM mode resonators are capacitively coupled by capacity coupling means provided separately, and coupling of the TEM mode resonators is achieved by combination of electromagnetic field coupling and capacity coupling. The dielectric filter of claim 12, wherein capacity coupling by the capacity coupling means is achieved in the second transmission lines. The dielectric filter of claim 12, wherein capacity coupling by the capacity coupling means is achieved at the open end of the TEM mode resonator. The filter of claim 12, 13 or 14, wherein the open end of the TEM mode resonator is grounded through the capacity coupling means. The filter of any of claims 12 to 15, wherein the attenuation pole of transmission characteristic is formed in a frequency range within 15% on both sides of the center frequency of the passband.

Inventors: AIZAWA KIMIO, FUJINO TAKASHI, ISHIZAKI TOSHIO, KUSHITANI HIROSHI, NAKAKUBO HIDEAKI, NAKAMURA TOSHIAKI, SASAKI ATSUSHI, SATOH YUKI, AIZAWA, KIMIO, FUJINO, TAKASHI, ISHIZAKI, TOSHIO, KUSHITANI, HIROSHI, NAKAKUBO, HIDEAKI, NAKAMURA, TOSHIAKI, SASAKI, ATSUSHI, SATOH, YUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: H01P1
IPC codes: H01P1